1. Field of the Invention
The present invention relates to an image data converter which converts YUV-format image data into RGB-format image data.
2. Related Art
Conventionally, in a case where an image is displayed on a CRT (Cathode Ray Tube), YUV-format image data (hereinafter referred to as YUV data) is converted into RGB-format image data (hereinafter referred to as RGB data). The YUV data is image data which represents an image as a luminous component (Y) and color difference components (U, V). The RGB data is image data which represents an image as red (R), green (G), and blue (B) components.
In general, a man's visual sense has sufficiently lower resolution characteristics with respect to the color difference components than with respect to the luminous component. By utilization of this fact, natural images are chiefly represented as the YUV data, whereby the amount of data is compressed by reducing the resolution of color difference signals to half the resolution of the luminance signal. In this way, the YUV data is compressed. The YUV data has several formats according to a compression rate of the color difference components. For example, according to YUV 422 (wherein four dots of the Y data correspond to two dots of the U and V data), color difference data regarding horizontally adjacent two dots is handled as the same data. Further, according to YUV411 (in which four dots of the Y data correspond to one dot of the U and V data), color difference data regarding a square area of two dots in a horizontal direction by two dots in a vertical direction is handled as the same data.
As described above, there are various types of YUV data. However, regardless of the form of the YUV data, there is a common algorithm for converting YUV data into RGB data. An equation for logically converting the YUV data to the RGB data is given by the following expressions (1) to (3). EQU R={(256/219).times.(Y-16)}+[{256/(224.times.0.713)}.times.(V-128)](1) EQU G={(256/219).times.(Y-16)}+[{(256.times.0.114)/(224.times.0.564.times.0.587 )}.times.(128-U)]+[{(256.times.0.299)/(224.times.0.713.times.0.587)}.times. (128-V)] (2) EQU B={(256/219).times.(Y-16)}.times.[{256/(224.times.0.564)}.times.(U-128)](3)
In general, when image data is transmitted to a CRT, the image data is converted by the above-described conversion expressions (1) to (3). The previously-described conversion of image data requires one of the following methods. One method is to convert the image data to be transmitted to the CRT from a YUV format to an RGB format using software or a general-purpose DSP (digital signal processor) before the image data is stored in video memory. Resultant RGB data is stored in the video memory. The thus-stored RGB data is sequentially transmitted to the CRT from the video memory in accordance with a display scanning operation.
However, according to this method, it is necessary for a MPU (microprocessor unit) or the like to convert the image data from the YUV format to the RGB format, thereby burdening the MPU. The YUV data comprises compressed UV components, and hence the RGB data has a larger amount of data than that of the YUV data. For this reason, a large amount of capacity of the video memory is needed accordingly.
Another method is to sequentially transmit the YUV data read from the video memory to the CRT after having converted it to RGB data using an arithmetic circuit such as a general-purpose DSP provided in a stage subsequent to the video memory.
However, according to this method, the conversion of YUV data to RGB data requires complicated calculation. Therefore, the scale of the arithmetic circuit provided so as to be subsequent to the video memory becomes larger.
To simplify the arithmetic circuit, there has already been proposed a method of converting YUV data to RGB data by carrying out arithmetic operations using an analogous equation. According to this method, for example, conversion of an R component is approximated by the following expression (4). ##EQU1##
By virtue of such approximation, the arithmetic circuit can be constructed by use of only simple shifters and full adders, which in turn enables a-reduction in circuit size.
Although the conventional approximation method allows a reduction in the circuit size in the manner as previously described, the converted R, G, and B components are different from their true values, thereby resulting in deterioration of picture quality of an output image.